The type of foundation your property has is one of the biggest factors in its susceptibility to subsidence. Older, shallower foundations are more vulnerable. Deeper, modern foundations are more resilient. But until you know what you're dealing with, you can't properly assess your risk or choose the right repair method.
This guide covers the foundation types most commonly found in Hertfordshire homes, their typical vulnerabilities, and which repair approaches work best for each.
Strip Foundations (1900s – 1970s)
The most common residential foundation type. A continuous strip of concrete (or sometimes brick) runs beneath every load-bearing wall.
- Typical depth: 450mm – 1m
- Very common in: Inter-war and post-war housing across Stevenage, Hatfield, Hemel Hempstead, and most Hertfordshire towns
- Subsidence vulnerability: High. These foundations sit within the "active zone" where clay soil moisture changes most dramatically. They're the foundation type most commonly affected by subsidence in Hertfordshire.
Repair options: Resin injection is highly effective for stabilising strip foundations on clay. For severe cases, mass concrete underpinning extends the foundation to a greater depth.
Trench Fill Foundations (1980s onwards)
A deeper, narrower trench filled entirely with concrete. Building regulations tightened in the 1980s, and trench fill became the standard for new construction, particularly in areas with known clay soil.
- Typical depth: 1m – 2.5m
- Common in: Properties built from the mid-1980s onwards
- Subsidence vulnerability: Low to moderate. The greater depth keeps the foundation below the most active zone of soil moisture change. However, even trench fill can be affected by large trees or exceptional droughts.
Repair options: Resin injection if needed, though these foundations rarely require intervention.
Raft Foundations
A thick concrete slab covering the entire footprint of the building. The load is spread across the full area rather than concentrated on strip lines.
- Typical depth: Surface level with thickened edges (typically 300mm slab, 450mm edges)
- Common in: Areas with soft or variable ground; some modern developments on former industrial land
- Subsidence vulnerability: Low. Raft foundations accommodate ground movement better because the load is distributed. However, they can still tilt if subsidence is severe and localised.
Repair options: Slab lifting with resin injection can re-level a raft foundation. Full underpinning is rarely needed.
Pad Foundations
Individual concrete pads supporting point loads, typically from steel or timber columns rather than walls.
- Common in: Extensions, conservatories, garages, and some commercial buildings
- Subsidence vulnerability: Variable. Depends entirely on pad size, depth, and ground conditions. Conservatories on shallow pads are frequently affected by subsidence.
Repair options: Resin injection to stabilise individual pads, or micro-piling for more significant loads.
Piled Foundations
Deep foundations that transfer loads to stable strata well below the surface — bypassing the problematic surface soils entirely.
- Typical depth: 3m – 15m+ (reaching chalk, gravel, or rock)
- Common in: Buildings on very poor ground, high-rise developments, and properties near rivers where surface soils are unsuitable
- Subsidence vulnerability: Very low. Piles reach ground that is unaffected by surface moisture changes.
Victorian and Pre-1900 Foundations
Many older Hertfordshire properties — particularly in historic towns like St Albans, Hertford, and Hitchin — have minimal foundations. Often just 2–3 courses of brick stepped out slightly wider than the wall above, sitting directly on compacted earth.
- Typical depth: 150mm – 300mm
- Subsidence vulnerability: Very high. These foundations have virtually no structural capacity to resist ground movement, and the building materials (lime mortar, soft brick) are more brittle than modern construction.
Repair options: Resin injection is often ideal — it stabilises the ground without the vibration and excavation of traditional underpinning, which could damage the existing structure further. For severe cases, mini-pile underpinning can extend foundations to stable depth.
Foundation Comparison Summary
| Foundation Type | Typical Depth | Era | Subsidence Risk | Best Repair Approach |
|---|---|---|---|---|
| Victorian/Pre-1900 | 150–300mm | Pre-1900 | Very high | Resin injection or mini-piles |
| Strip | 450mm–1m | 1900–1970s | High | Resin injection or mass concrete underpinning |
| Trench fill | 1–2.5m | 1980s onwards | Low-moderate | Resin injection (rarely needed) |
| Raft | Surface | Variable | Low | Slab lifting with resin |
| Pad | Variable | Variable | Variable | Resin injection or micro-piling |
| Piled | 3–15m+ | Modern | Very low | Rarely needed |
How to Find Out What Foundation You Have
Unless you have the original building plans (check with your local authority's Building Control department), you probably don't know your foundation type. Options for finding out:
- Building Control records — your local council may hold plans showing foundation details, particularly for properties built after 1965
- Trial pit — a small excavation alongside the foundation reveals its depth and construction
- Ground-penetrating radar — a non-invasive survey that can image the foundation without digging
Knowing your foundation type is the first step in understanding your property's subsidence risk. If you're unsure and want a professional assessment, we offer free surveys across Hertfordshire.